In recent years, accompanying the popularization of portable information terminals and digitalization of information, there has been an increasing demand for information recording/reproducing devices or memory devices, that are compact in size and have a large capacity. Especially, NAND type flash memory and compact HDD (Hard Disk Drive) have increased recording density due to the advance of miniaturization technique, and are adopted in various types of devices. However, further increases in recording density and capacity are being demanded. That is, miniaturization, high density and high speed are further required which are unrealizable with conventional techniques. Accordingly, a resistance varying type nonvolatile semiconductor memory configured to record different electrical resistance values as information is receiving attention as an improved device.
The resistance varying type nonvolatile semiconductor memory is configured by a variable resistance layer and electrodes, the electrodes sandwiching the variable resistance layer. The variable resistance layer can take two or more different electrical resistance states, for example, a low-resistance state and a high-resistance state. In the nonvolatile semiconductor memory, the resistance state of the variable resistance layer is caused to change by applying a voltage, current, or charge not less than threshold value between the electrodes, the difference in resistance value being recorded corresponding to data. Furthermore, the nonvolatile semiconductor memory has a feature that the resistance state is nondestructively readable by applying a voltage, current, or charge not more than the threshold value.
Currently undergoing research and development as an element material for the resistance varying type nonvolatile semiconductor memory are multi-component metal oxides such as nickel oxide (NiO) or strontium zirconium oxide (SrZrO3). However, there are difficulties in controlling composition and crystalline structure to manufacture the metal oxides. Moreover, characteristics of the metal oxides are unstable, which makes it difficult to realize desired electrical characteristics of the metal oxides with good reproducibility. Consequently, although R&D for a suitable material for the variable resistance element is under way, an optimal material has yet to be found.
Furthermore, carbon-based materials are also subject to R&D as candidates for the variable resistance layer, and their manufacturing methods. Carbon-based materials are configured by single carbon, and hence have the merit that control of composition is comparatively easy and easily controllable with little dependence on process conditions. However, unless manufactured at high temperature and high pressure, a carbon film tends to be formed as black lead or a so-called graphite structure, leading to low resistivity. Varying of the resistance state is considered to be caused by change in bonding state, that is, change between sp3 bond and sp2 bond, of carbon in the film. Accordingly, a large current is required to change the bonding state.
When a non-volatile memory is configured by arranging memory cells in a matrix, and/or by stacking the memory cells in an integration manner, wirings connected to the memory cells are necessarily long. If an operating current flowing in an individual memory cell in such a memory is large, the voltage drop in the wirings themselves becomes significant. As a result, it becomes impossible to supply the memory cells with the voltage required in an operation. Furthermore, if the wiring resistance is large, a signal caused by change in resistance of the memory cell cannot be detected with high accuracy. That is, as an operation current flowing in an memory cell becomes larger, the size of a cell matrix, and the number of accumulation layers should be smaller. Accordingly, a memory device with a large cell operation current is not suited for high integration. In addition, there is also the problem that current consumption of the device increases.